There are a number of applications where a high frequency equalization circuit is needed to provide a signal rising or falling in amplitude with frequency over a plurality of possibly overlapping operating ranges each operating range covering a range of frequencies and adjustable within each range. One significant area of application is in signal recorders and especially audio recorders. Other applications exist such as plural range tone controls.
Due to the signal transfer characteristics of present audio recorders, and especially the characteristics of recording tape, the high frequency range of the audio signal becomes attenuated by the recording and reproduction process. This attenuation occurs to varying degrees dependent upon a number of variables including relative transducer to record medium speed and the frequency characteristics of the tape. Consequently, an adjustable high frequency amplitude equalization circuit is commonly employed in the audio record circuitry of the recorder to compensate for the attenuation. The audio equalization circuitry may be designed into the audio preamplifier.
In the past, passive RC filters utilizing adjustable capacitors have been used with limited success as adjustable amplitude equalizers. For example, many multiple speed audio tape recorders have tape speed capabilities ranging from 33/4 inches per second to 30 ips. Recorders having a wide range of selectable operating speeds have required for each speed a separate filter with a variable capacitor. While such passive equalizers effect equalization, they do so at the expense of appreciable overall signal attenuation and concomitant loss of signal power and quality. Because the filters require a large number of elements and a variable capacitor for each tape speed, the passive equalizers are expensive.
It is preferable to utilize an active filter to minimize signal attenuation. The use, however, of a separate active filter for each tape speed in a multiple speed machine can increase expenses beyond the cost of using separate passive filters.
Many filter circuits, both passive and active, can introduce a significant amount of noise into the signal. Many introduce noise when the equalization provided by the circuit is adjusted to zero. The same level of noise may be introduced regardless of the amount of equalization provided. Active filters, moreover, may introduce noise into the system inversely to the level of signal amplitude enhancement or equalization introduced. This later case is especially undesirable because the signal to noise ratio becomes high with small amounts of equalization.